One of the widely used laser markers that provide a laser beam for marking characters or figures on workpieces, such as semiconductor packages, is a so-called liquid crystal laser marker using a liquid crystal mask. In the liquid crystal laser marker, a design of the characters or figures to be marked is converted to dot information, e.g., where "0" indicates an unmarking portion and "1" indicates a marking portion, and is displayed on the liquid crystal mask having a given number of pixels in accordance with the dot information. Then a laser beam is directed to be incident on the liquid crystal mask and is transmitted through the pixels, corresponding to the characters or figures, to the surface of a workpiece so that the workpiece will be marked with the transmitted laser beam. Therefore, the more the number of pixels of the liquid crystal mask increases per unit area of the workpiece, the more the characters or figures can be marked smoothly with high resolution.
A conventional marking method for such a liquid crystal laser marker will be described with respect to FIGS. 14 to 17.
FIG. 14 is a schematic diagram showing an exemplary structure of the liquid crystal laser marker. In FIG. 14, the laser oscillator 21 can be a YAG laser oscillator, the output of which is a pulse train driven by a Q switch. A first deflector 23X, 23Y conducts the laser beam from the laser oscillator 21 to a liquid crystal mask 2 while deflecting it in given X and Y directions so that the liquid crystal mask 2 will be raster scanned. The first deflector comprises of an X-directional deflector or polygonal mirror 23X and a Y-directional deflector or galvanometer scanner 23Y, provided separately from each other. Such scanning directions, i.e., the X-axis scanned by rotating the polygonal mirror 23X and the Y-axis scanned by moving the galvanometer scanner 23Y, intersect at right angles. The polygonal mirror 23X is rotatable in several constant speed rotation modes, and the rotation mode is selected for each workpiece to be marked. Each plane of the polygonal mirror 23X corresponds to one line in the X-direction on the liquid crystal mask 2. On the other hand, the galvanometer scanner 23Y is operable with given steps of minute, equiangular deflections, and moves only by a minute deflection angle and stops there as the light-receiving point of the laser beam from the galvanometer scanner 23Y is changed from one plane to another by the rotation of the polygonal mirror 23X. Each of the minute deflection angles corresponds to one line feed in the Y-direction on the liquid crystal mask 2.
As shown in FIG. 15, the liquid crystal mask 2, called a transmission dispersion type liquid crystal mask, can be a liquid crystal device in which a given number of liquid crystal elements are arranged in a dot matrix. The liquid crystal elements constituting the liquid crystal mask 2 have the same size. Further, electrode lines, not shown, are arranged on both sides of the liquid crystal elements so as to be parallel on each side and to perpendicularly intersect between both sides. The electrode lines apply a given voltage to certain or specific liquid crystal elements to place them in a laser-beam transmission state. The other liquid crystal elements to which no voltage is applied are in a laser-beam scattered state. The liquid crystal elements arranged in the dot matrix are used as the pixels 3 of a design to be marked. Each of the pixels 3 is converted to dot information "0" or "1", where "0" indicates an unmarking portion and "1" indicates a marking portion. The given voltage is applied to the pixels 3 corresponding to the portions to be marked such dot information is displayed on the liquid crystal mask 2, so that the laser beam will be transmitted therethrough, thus marking a workpiece with the transmitted laser beam. As discussed above, the liquid crystal mask 2 serves as a light shutter for transmitting or intercepting a light beam in response to an external signal.
A second deflector 27X, 27Y conducts the laser beam from the liquid crystal mask 2 to a marked surface of a workpiece 30 and deflects it in given X and Y directions on the workpiece 30. The second deflector comprises an X-directional deflector or galvanometer scanner 27X and a Y-directional deflector or lens system 27Y, provided separately from each other. Such directions, i.e., the X-axis in which the laser beam is deflected by rotating the galvanometer scanner 27X and the Y-axis in which the laser beam is deflected by moving the lens 27Y in parallel to the workpiece 30, intersect at right angles. The laser beam transmitted through the liquid crystal mask 2 is conducted to the workpiece 30 via the galvanometer scanner 27X for X-directional deflection, an object lens 28, and the lens 27Y for Y-directional deflection. Thus, the design displayed on the liquid crystal mask 2 is marked on the workpiece 30. The lens 27Y for Y-directional deflection is set in a hole of a table 36 and is moved along with the table 36 in parallel to the surface of the workpiece 30. The table 36 is coupled through a link mechanism to an output shaft of a drive unit 35, such as an AC motor, and is driven by the drive unit 35 to move in parallel. The second deflector 27X, 27Y is in a stopped state until all of the pixels corresponding to the design on the liquid crystal mask 2 are raster scanned so that the laser beam transmitted through the liquid crystal mask 2 can be directed to a marking area of the design.
Further, optical systems for gathering or condensing laser light are arranged within incidence paths to the first deflector 23X, 23Y and to the second deflector 27X, 27Y, respectively. Such condenser optical systems are effective in condensing incident light when the beam has large diameter or deflection angle, thus reducing any occurrence of modification of the mark or dispersion or loss of laser light.
The condenser optical system for the first deflector 23X, 23Y is constituted of a relay lens (e.g., beam splitter) 22 between the laser oscillator 21 and the galvanometer scanner 23Y, and a relay lens 24 between the galvanometer scanner 23Y and the polygonal mirror 23X. The relay lens 22 gathers or condenses the laser beam from the laser oscillator 21 onto the reflection plane of the galvanometer scanner 23Y, while the relay lens 24 gathers the deflected beam from the galvanometer scanner 23Y to a point on each plane of the polygonal mirror 23X. The laser beam from the polygonal mirror 23X to the liquid crystal mask 2 is thus made uniform for raster scanning.
The condenser optical system for the second deflector 27X, 27Y is constituted by a relay lens (e.g., field lens) 25, arranged between the polygonal mirror 23X and the galvanometer scanner 27X and close to the liquid crystal mask 2. The relay lens 25 alters the raster-scanned beam from the polygonal mirror 23X into parallel rays, and the galvanometer scanner 27X reflects the parallel rays. The relay lens 25 can be arranged on the incidence side of the liquid crystal mask 2, as shown in FIG. 14, or on the emission side or on both sides.
A controller 11 is such a computer system that mainly has a microcomputer. The controller 11 is connected to the liquid crystal mask 2, the drive unit 32 for the polygonal mirror 23X, the Q switch for the laser oscillator 21, the drive unit 31 for the galvanometer scanner 23Y, the drive unit 34 for the galvanometer scanner 27X, and the drive unit 35 for the lens 27Y, respectively. These elements or units are controlled by the controller 11.
FIG. 16 shows an exemplary design to be marked. When the design is larger than the number of pixels on the liquid crystal mask 2, marking with one emission of the laser beam causes lowered resolution of the marked design, and hence rough characters or lines. In such a case, it is necessary to use the liquid crystal mask 2 several times for a reduced design area (hereinafter, referred to as a block) to be marked with one emission of the laser beam. For example, the overall design is divided into two blocks 1 in lateral and vertical directions, respectively. Then a design portion corresponding to each respective divided block 1 is displayed on the liquid crystal mask 2, and is marked in position on the workpiece 30 one by one. After all of the individual blocks 1 have been marked, the blocks 1 are synthesized and the overall design is completely marked.
Stated more particularly, a block 1 is first selected from among all of the blocks 1 during the condition that the oscillating power of the laser oscillator 21 is turned off by the Q switch. The design portion of the selected block 1 is displayed on the liquid crystal mask 2 as dot information "0" and "1". The first deflector 23X, 23Y is driven to move to and stop at a position from which the raster scanning is started, while the second deflector 27X and 27Y is driven to move to and stop in a marking area corresponding to the marking position of the selected block 1. Then the laser oscillator 21 is turned ON by the Q switch to output pulses, and the first deflector 23X, 23Y is driven so that the design portion on the liquid crystal mask 2 will be raster scanned. The selected block 1 is thus marked on the workpiece 30.
The above process is performed for the other blocks 1 sequentially and repeatedly until the overall design is marked on the workpiece 30.
In the conventional art, a design portion of each block 1 is displayed on the liquid crystal mask 2 and is marked on the workpiece 30 as discussed above.
However, with such a conventional technique, a character or figure may be divided into different blocks 1. In this case, the identical character or figure must be marked on the workpiece 30 several times.
It is therefore necessary to coincide the joints of adjacent blocks with each other when marking the identical character or figure several times, but such an adjustment is difficult because of various factors, such as a play of the second deflector 27X and 27Y, the changes in the position of optical elements, and a change of a control signal due to external noise, and a gap may occur in the joint between adjacent blocks. In FIG. 16, a character "I" is divided into two parts. FIG. 17A shows a state where the divided character "I" has been marked normally, whereas FIG. 17B shows the character "I" marked in the condition that a gap occurs in the joint between the two blocks 1. As shown in FIG. 17B, the gap between the blocks 1 causes a portion in which no laser beam is irradiated, and hence a break in the character or figure. As a result, a problem arises in that the visibility of the marked design is lowered.